Transducers such as ultrasonic transducers are provided in a wide variety of electronic applications. As the need to reduce the size of many components continues, the demand for reduced-size transducers continues to increase as well. This has lead to comparatively small transducers, which may be micromachined according to technologies such as micro-electromechanical systems (MEMS) technology. One type of transducer is a piezoelectric micromachined transducer (PMT). The PMT includes a layer of piezoelectric material between two conductive plates (electrodes) thereby forming a membrane. When functioning as a receiver, an acoustic wave incident on the membrane results in the application of a time varying force to the piezoelectric material. Application of the time-varying force to a piezoelectric material results in induced stresses in the piezoelectric material, which in-turn creates a time-varying voltage signal across the material. This time-varying voltage signal may be measured by sensor circuits to determine the characteristics of the incident acoustic wave. Alternatively, this time-varying voltage signal may produce a time-varying charge that is provided to sensor circuits that process the signal and determine the characteristics of the incident acoustic wave. When functioning as a transmitter, a voltage excitation produces vibration of the diaphragm. This in turn radiates acoustic energy into the air (or any gaseous medium).
Ultrasonic devices, such as ultrasonic transducers, typically operate at a resonance condition to improve sensitivity in both receive mode and transmit mode. Accordingly, it is useful for the transducer to function at a comparatively accurate resonant frequency, and for multiple transducers designed for use at the selected resonant frequency to be fabricated with such accuracy with repeatability. One drawback to many known PMT structures relates to a lack of repeatability of the resonance frequency from PMT to PMT. To this end, PMTs for certain applications, such as mics rely on the flexure mode of the membrane rather than longitudinal modes. While the resonant frequency of longitudinal modes is not significantly affected by film stress, the resonant frequency of flexural modes is highly dependent on film stress. Thus, variation in film stress can impact the operational characteristics of transducers designed for flexural mode operation.
Another source of stress in PMTs is temperature. As is known, every material has a coefficient of thermal expansion (TCE). Thus a material expands or contracts in proportion to this coefficient. The expansion or contraction of a material induces stress in the material, and mismatches in TCE between different materials comprising the PMT will result in stress in the membrane layer. The stress in the membrane layer can impact the resonance frequency and the sensitivity of the membrane and thereby the PMT.
There is a need, therefore, for a transducer structure that addresses at least the shortcomings described above.